This application claims the priority of Japanese Patent Application No. 2000-084488 filed on Mar. 24, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a diffraction type lens in an optical pickup apparatus commonly usable for at least two kinds of optical recording media, in which, when respective irradiation light beams for the optical recording media have wavelengths different from each other according to the kinds of optical recording media, each light beam can efficiently be converged onto its corresponding optical recording medium; and an optical pickup apparatus using the same.
2. Description of the Prior Art
Various kinds of optical recording media have recently been developed, and optical pickup apparatus which can carry out recording and reproducing while commonly using a plurality of kinds of optical recording media have been known. For example, apparatus carrying out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by use of an optical pickup have been known.
As for such two kinds of optical recording media, while visible light at about 635 nm is used for DVD in order to improve the recording density, infrared light at about 780 nm is necessary for CD-R since it is insensitive to light in the visible region. Therefore, optical pickup apparatus which can commonly be used for both of them are based on a so-called dual wavelength beam system.
If the disc thickness value varies between the above-mentioned two kinds of optical recording media, however, then such an optical pickup apparatus is required to exert different converging actions on the respective wavelengths of light for reproducing or recording.
Though a system in which two objective lenses having respective converging actions different from each other are exchangeable according to optical recording media to be reproduced or recorded has been known to respond to such a requirement, it complicates the structure of optical pickup apparatus, thereby contradicting a demand for compactness and lower cost.
On the other hand, though optical pickup apparatus using a parallel-plane diffraction optical device or the like as an objective lens thereof have been known, none of them satisfies the demand for using two wavelengths of light for the reproducing or recording of their corresponding optical recording media having disc thickness values different from each other japanese Unexamined Patent Publication Nos. 61-287042, 8-62493, and 7-191219, and the like).
In view of the circumstances mentioned above, it is an object of the present invention to provide a compact, inexpensive diffraction type lens which can favorably converge two wavelengths of light onto their corresponding optical recording media having disc thickness values different from each other, without complicating the structure of optical pickup apparatus.
The present invention provides a diffraction type lens, disposed in a luminous flux, having a wavelength selectivity;
wherein a substrate constituting the lens has one surface formed with a zone plate exhibiting a smaller converging action with respect to a first wavelength xcex1 of light and a greater converging action with respect to a second wavelength xcex2 of light, and the other surface formed with a zone plate exhibiting a smaller converging action with respect to the second wavelength xcex2 of light and a greater converging action with respect to the first wavelength xcex1 of light, the substrate being transparent to the first and second wavelengths xcex1 and xcex2 of light; and
wherein each of the zone plates comprises concentric gratings having a stepped cross section.
Preferably, the diffraction type lens has a parallel-plane form.
Each of the stepped concentric gratings has three steps, for example.
Preferably, one surface formed with the zone plate has a stepped portion with a height h1 per step satisfying the following conditional expressions (1) and (2), and the other surface formed with the zone plate has a stepped portion with a height h2 per step satisfying the following conditional expressions (3) and (4):
h1=L1xcex1/(n1xe2x88x921)xe2x80x83xe2x80x83(1)
h1=M1xcex2/(n2xe2x88x921)+K1xcex2/2(n2xe2x88x921)xe2x80x83xe2x80x83(2)
h2=L2xcex2/(n2xe2x88x921)xe2x80x83xe2x80x83(3)
h2=M2xcex1/(n1xe2x88x921)+K2xcex1/2(n1xe2x88x921)xe2x80x83xe2x80x83(4)
where
xcex1 and xcex2 are two wavelengths of incident light;
n1 is the refractive index of grating portion with respect to the first wavelength xcex1 of light;
n2 is the refractive index of grating portion with respect to the second wavelength xcex2 of light;
L1 and L2 are positive integers;
M1 is the maximum value among 0 and positive integers satisfying the conditional expression of h1 greater than M1xcex2/(n2xe2x88x921);
M2 is the maximum value among 0 and positive integers satisfying the conditional expression of h2 greater than M2xcex1/(n1xe2x88x921); and
K1 and K2 are 0.27xe2x89xa6K1, K2xe2x89xa60.73, or 1.27xe2x89xa6K1, K2xe2x89xa61.73.
Also, the present invention provides an optical pickup apparatus comprising the above-mentioned diffraction type lens, wherein the luminous flux incident on the diffraction type lens is a substantially parallel luminous flux.
Preferably, in the optical pickup apparatus, the luminous flux is converged at a position where two kinds of optical recording media having respective thickness values different from each other are disposed, one optical recording medium being recorded or reproduced with the first wavelength xcex1 of light, the other optical recording medium being recorded or reproduced with the second wavelength xcex2 of light.